1. Field of the Invention
The present invention relates in general to an anti-lock or anti-skid brake system, and more particularly to improvements in a technique for controlling a flow of a brake fluid from a hydraulic power source to a wheel cylinder for applying brake to a wheel.
2. Discussion of the Prior Art
In the art of applying brake to a wheel of a vehicle, an anti-lock braking device for preventing a locking or skidding condition of the vehicle wheel is known. A typical brake system having such an anti-lock braking device generally includes a master cylinder for generating a pressurized fluid, a wheel cylinder for applying brake to the wheel, a reservoir for accommodating the brake fluid discharged from the wheel cylinder, and a solenoid-operated valve device disposed between the master cylinder and the wheel cylinder and between the wheel cylinder and the reservoir. The pressure of the fluid delivered from the master cylinder varies with an operating amount or force of a brake pedal or other operating member. The solenoid-operated valve device is controlled for automatic regulation of the pressure in the wheel cylinder.
The solenoid-operated valve device may be a three-position valve having a pressure-increase position, a pressure-decrease position, and a pressure-hold position. In the pressure-increase position, the valve device disconnects the wheel cylinder from the reservoir and permits fluid communication of the wheel cylinder to the master cylinder. In the pressure-decrease position, the wheel cylinder is disconnected from the master cylinder and is held in fluid communication with the reservoir. In the pressure-hold position, the wheel cylinder is disconnected from both the master cylinder and the reservoir. Alternatively, the solenoid-operated valve device may be a two-position valve which does not have the pressure-hold position. Further, the valve device may be a combination of two or more valves, for example, two solenoid-operated shut-off valves, one of which is disposed between the master and wheel cylinders, and the other of which is between the wheel cylinder and the reservoir. Another example of the combination valve device consists of a solenoid-operated directional control valve and a solenoid-operated shut-off valve.
The solenoid-operated valve device of any type is controlled by an electric control device utilizing a computer. Generally, the control device controls the valve device such that the valve device is normally placed in the pressure-increase position permitting the wheel cylinder pressure to increase, and such that the valve device operates in an anti-lock state when there arises a skidding or locking of the vehicle wheel. In the anti-lock state, the pressure in the wheel cylinder is alternately increased and decreased with the valve device placed alternately in the pressure-increase and pressure-hold positions, so that the slip ratio of the wheel is maintained within a predetermined range.
It is recognized that the rate or gradient of the pressure increase of the wheel cylinder during an anti-lock operation of the valve device is preferably regulated for optimally or accurately controlling the wheel cylinder pressure. To this end, it is proposed to suitably determine the time intervals at which the valve device is switched between the pressure-increase position and the pressure-decrease position (or pressure-hold position). It is also proposed to adjust the time length during which the valve device is held in the pressure-increase position in each control cycle, as needed. These proposals are not completely satisfactory in attaining a sufficient result in terms of the pressure control accuracy of the valve device operating in the anti-lock state. Specifically, upon switching of the valve device into the pressure-increase position during the anti-lock operation, the rate of flow of the brake fluid through the valve device significantly varies depending upon a pressure difference between the pressures on the upstream and downstream sides of the valve device. In other words, the amount of increase in the wheel cylinder pressure per unit time of operation in the pressure-increase position considerably fluctuates with the above-indicated pressure difference.
In light of the above drawback, the assignee of the present application developed an anti-lock brake system having a recirculating type anti-lock braking arrangement which is provided with a hydraulic pump, in addition to the master cylinder, wheel cylinder, reservoir, solenoid-operated valve device and electric control device, which have been described above. The pump is activated upon commencement of the anti-lock operation of the valve device in the anti-lock state, so that the fluid is returned by the pump from the reservoir back to the master cylinder. This brake system includes a pilot-operated variable flow control device which is disposed in a fluid passage between the master and wheel cylinders. The flow control device is adapted to receive as pilot pressures the master cylinder pressure and the wheel cylinder pressure, and operate so as to reduce the area of fluid flow therethrough (amount of fluid flow through the fluid passage) toward the wheel cylinder. The fluid flow is reduced with an increase in the difference between the two pilot pressures, and with an increase in the wheel cylinder pressure. The brake system of this type is described as one embodiment of the invention disclosed in Japanese Patent Application No. 63-45423 (which had not been laid open at the time of the priority date claimed in this application), the subject matter of which is disclosed in U.S. patent application Ser. No. 307,481 filed Feb. 8, 1989 (assigned to the assignee of the present application) which issued as U.S. Pat. No. 4,957,330 to Morikawa et al.
The brake system referred to above performs an anti-lock braking operation in which the rate of fluid flow from the master cylinder toward the wheel cylinder is suitably regulated when the solenoid-operated valve device in the anti-lock state is switched into the pressure-increase position. Namely, the fluid flow into the wheel cylinder upon switching of the valve device into the pressure-increase position is accurately controlled so that the wheel cylinder pressure is raised at an optimum rate suitable for the anti-lock braking operation, irrespective of the level of the wheel cylinder pressure which should correspond to the friction coefficient of the road surface, and the level of the master cylinder pressure which corresponds to the operating amount or force of the brake operating member.
However, the present applicants recognized a further room for improvement of the brake system discussed above, in connection with the manner of operation when the brake operating member is operated either comparatively slowly and abruptly. Described more particularly, where brake is slowly applied to the wheel by a relatively slow operation of the operating member, the wheel cylinder pressure is raised such that the wheel cylinder pressure is held substantially equal to the master cylinder pressure. In this instance, there arises substantially no pressure difference between the two pilot pressures of the variable flow control device. Where the braking to the wheel occurs relatively abruptly with a relatively rapid operation of the operating member, on the other hand, the wheel cylinder pressure tends to be considerably lower than the master cylinder pressure, due to a resistance to the flow of the fluid through the flow control device. In a normal braking operation with the solenoid-operated valve device held in the pressure-increase position, it is required that the rate of flow of the fluid into the wheel cylinder should be sufficiently high. However, the rapid operation of the operating member causes considerable reduction in the area of fluid flow through the flow control device, because of the considerably large difference between the two pilot pressures, whereby the rate of fluid flow into the wheel cylinder is insufficiently high. As a result, the actual braking effect is delayed, or the response of the brake system to the operation of the operating member is deteriorated.